Due to the increasing recording density of discs, a spindle motor used in a disc apparatus is required to rotate a disc while holding it precisely, and a disc apparatus is requested to further increase its transfer rate. Accordingly, the disc must be rotated at a high speed.
A configuration of a conventional general spindle motor S is described with reference to the side view in FIG. 8. The spindle motor S comprises a rotor 4 mounted on a fixed portion 10 via a rotating shaft 5 and a turn table 3 (for example, a molding of a synthetic resin) attached to the top of the rotating shaft 5. The rotor 4 is press-fitted and fixed to the rotating shaft 5, and the turn table 3 is also press-fitted and fixed to the rotating shaft 5. In this configuration, a disc 1 is rotated while being sandwiched between the turn table 3 and a clamper 2.
With reference to the block diagram for rotation control in FIG. 9, the control of the spindle motor S in an actual disc apparatus D is described.
A rotation control section 21 drives the spindle motor S using motor driving 22 based on a linear speed instruction 20. A head 24 reproduces a signal from the disc 1, and a linear speed calculation means 26 determines a linear speed 27 from the head reproduced signal 25 as a feedback signal to a rotation control section 21. In this manner, the spindle motor S executes so-called CLV control that uses a signal reproduced from the head to control the rotational speed of the disc 1 in such a way that a linear speed 27 is constant.
In the conventional spindle motor S in which the rotor 4 and the turn table 3 are each press-fitted to the rotating shaft 5, the rotating shaft 5 constitutes the only coupled portion between the turn table 3 and the rotor 4, so the rigidity between these components is weak, resulting in unwanted vibration in the path shown by arrow C as shown by the imaginary lines in FIG. 8.
That is, if the turn table 3 and the rotor 4 are coupled together by simply press-fitting them to the rotating shaft 5, then as they are rotationally driven, whirling occurs in which they move relatively.
The unwanted vibration caused by whirling adversely affects the rotation control characteristics of the spindle and the focus control characteristics of a pickup, thereby making each control system unstable. The effects of this unwanted vibration can be confirmed by measuring the loop characteristic of the control system. This is described with reference to FIG. 9 and the characteristic diagram in FIG. 10 showing a CLV loop of the spindle motor S. FIG. 10 shows a transfer function using the linear-speed instruction 20 as input and the linear speed 27 as output, wherein the horizontal axis indicates the frequency (Hz) while the vertical axis indicates the gain (dB) and phase (deg) of the loop characteristic.
As is apparent from FIG. 10, the gain increases near the frequency of 800 Hz. This is caused the above unwanted frequency, and when the conventional spindle motor S is used to increase the loop gain, the system may oscillate at the frequency of 800 Hz, thereby preventing the follow-up characteristic of disc rotation control from being improved.
The molding thickness of the turn table 3 is reduced to improve the molding accuracy. The reduced thickness of the apparatus limits its height and thus the height of the turn table 3, thereby reducing the rigidity of the turn table. Furthermore, if the bearing of the spindle motor S is a sliding bearing. the diameter of the rotating shaft 5 is reduced to reduce the peripheral speed of the surface of the bearing in order to improve the lifetime expectancy of the bearing for fast rotations. In this case, the rigidity of the rotating shaft 5 decreases.
If the rigidity between the turn table 3 and the rotor 4 decreases as described above, the unwanted vibration is further increased by the vibration mode in which the rotor section 4 and the turn table section 3 including the disc 1 move relatively.
Japanese Patent Application Laid-Open No. 8-195010 discloses a technique for holding the disc 1 precisely. This application discloses a disc chucking mechanism having a centering function.
In addition, Japanese Patent Application Laid-Open No. 9-63164 discloses a technique using a spindle motor S having a disk chucking function in the turn table 3 to account for the reduced thickness and size of the apparatus. In this case, claws provided on the turn table 3 are used to chuck the disc. Either technique, however, is insufficient to restrain the unwanted vibration.
Thus, Japanese Patent Application Laid-Open No. 7-298586 discloses a configuration in which the rotor 4 of the spindle motor S and the turn table 3 are integrated together by means of adhesion or welding and in which the rotating shaft 5 is press-fitted to the turn table 3.
In the spindle motor S disclosed in Japanese Patent Application Laid-Open No. 7-298586, the rotor 4 of the spindle motor S and the turn table 3 are integrated together by means of adhesion or welding, so this configuration is subjected to few effects of the vibration mode in which the turn table 3 and the rotor 4 move relatively.
Since, however, the deflection accuracy of the rotor 4, however, is the sum of the deflection of the turn table 3 and the surface accuracy of the coupled portion between the rotor 4 and the turn table 3, an insufficient shaft deflection accuracy causes the gravity of the rotor 4 section to be biased to increase the unwanted vibration when the spindle motor S is rotated at a high speed.
In addition, the method for welding the turn table 3 has an advantage of eliminating the need to apply an adhesive but has a disadvantage of causing hot distortion (the deformation of resin during a welding process) during welding, thereby hindering the accuracy of the turn table 3 from being controlled.